Polyelectrolyte complexation (PEC) is an associative phase separation process initiated by mixing of two kinds of oppositely charged polyelectrolyte solutions. PEC-based materials have been recognized as ideal prototypes to study membraneless organelles, self-assembled into smart delivery vehicles for nucleotides and proteins, and fabricated into functional devices for purification purposes. Continuous studies have been devoted to understanding thermodynamics, kinetics, morphology, and structures of the polyelectrolyte complexation systems, which were summarized in Chapter 1 of this dissertation. However, a lot of the studies remained qualitative, and the fundamental understanding of the polymer physics involved has not kept pace. Specifically, a precise description of the binodal phase diagram capturing detailed phase behaviors and partitioning of different components into the two respective phases is still lacking. Moreover, joint efforts by experiments and theories are needed to provide an overarching framework with the predictive capacity to control the properties of the PEC-based materials. Accordingly, we have designed a new experimental approach combining salt resistance measurement and thermogravimetric analysis with minimum processing and approximation to produce accurate binodal phase diagrams. Chapter 2 of this dissertation focused on illustrating the phase behaviors of a “clean” polypeptide system, poly(lysine) and poly(glutamic acid) with hydrophilic backbones and matched chain lengths. Essential features of this system were demonstrated, including the screening effect of salt, self-suppression and the change of the volume fraction of the complex phase with addition of salt. Additionally, the salt partitioning into the supernatant phase was found to initially increase and then decrease on increasing the salt concentrations, manifesting as a distinct minimum in the salt partition coefficients. These trends were shown by simulations to be strongly influenced by the excluded volume interactions in the complex phase, which were not accounted for in their entirety in earlier theories. Equipped with the knowledge from the polypeptide system, we have further extended our research to an aliphatic pair of poly(acrylic acid) and poly(allylamine hydrochloride) with hydrophobic backbones in Chapter 3 of the dissertation. While we found the phase behaviors were, to some extent, similar under neutral/basic pHs and followed general expectations of PEC materials, remarkably different behaviors were observed under acidic conditions. Unintuitively, polymer content in the complex phase increased as salt was added, due to the hydrophobicity of aliphatic polymer backbones coupled with hydrogen bonding of unionized monomer units. We systematically investigated both of these specific interactions using turbidimetry, microscopy, and FTIR spectroscopy. These binodal phase diagrams detailed the associative secondary phase assembly from electrostatic complexation, precipitation of an individual polymer, and other noncovalent contributions as a function of pH, polymer concentration, and added salt. Lastly, Chapter 4 of the dissertation has compared the binodal phase behaviors of both polypeptide and aliphatic polyelectrolyte systems to accentuate the effect of solvent quality and chain length. The experimental results could be qualitatively explained by the scaling theory and random phase approximation calculation. The threshold value of Flory-Huggins χ parameter distinguishing complexation disintegration upon salt addition and that remaining stable even at high salt concentrations could be estimated. Additionally, the salt resistance concentration was shown to be proportional to the square root of the polymer chain length.